using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Bictionary<string, int> bictionary = 

            new Bictionary<string,int>() {

                { "3",1 }, 

                { "b",2 }, 

                { "c",3 } 

            };

        // test forward lookup

        Console.WriteLine(bictionary["b"]);

        Console.WriteLine(bictionary["3"]); 

        // test forward lookup error

        //Console.WriteLine(bictionary["d"]);

        // test reverse lookup

        Console.WriteLine(bictionary[3]); // keys on data type and value, not just value

        // test reverse lookup error (throws same error as forward lookup does)

        Console.WriteLine(bictionary[4]); 

    }

}

​

public class Bictionary<T1, T2> : Dictionary<T1, T2>

{

    public T1 this[T2 index]

    {

        get

        {

            if(!this.Any(x => x.Value.Equals(index)))

               throw new System.Collections.Generic.KeyNotFoundException();

            return this.First(x => x.Value.Equals(index)).Key;

        }

    }

}